Re: What's in a profile?
Re: What's in a profile?
- Subject: Re: What's in a profile?
- From: Marco Ugolini <email@hidden>
- Date: Sun, 23 Jul 2006 20:49:26 -0700
In a message dated 7/23/06 7:34 PM, Lee Blevins wrote:
> Is there a dummies guide to the structure of an icc profile?
>
> Something dumbed down so someone with my limited experience can
> understand it.
There is an Appendix A to Bruce Fraser, Chris Murphy and Fred Bunting's
"Real World Color Management" (I have the 2003 edition). It's called
"Profile Anatomy". It talks about the main components of an ICC profile (ICC
v2 in that case: some things may have changed with ICC v4, I'm not sure):
Profile Header, Tag Table, and the Tagged Elements Data (commonly referred
to as "profile tags").
Then there is Abhay Sharma's "Understanding Color Management", which
dedicates Chapter 5 ("Inside Profiles") to a description of the same general
subject.
> I would imagine at some location in the file there is a table of input
> colors and output colors. Am I right in this assumption or is it a
> formula instead of a lookup table?
>
> Does it use both possibly?
There are:
- matrix-based profiles (using the type of matrices you learn of in linear
algebra, usually 3x3): these are found in RGB working profiles like
AdobeRGB, ColorMatch RGB and ProPhoto RGB;
- and then there are profiles that are based on LUTs (Look-Up Tables, 2 per
rendering intent: one from source profile to the Profile Connection Space,
i.e. A2B; the other from the PCS to the target profile, i.e. B2A). CMYK
output profiles are LUT-based (e.g., US Web Coated (SWOP) v2, US Sheetfed
Coated v2, etc.).
But it's worthy of mention to note that there is one RGB working space that
I know of, PhotoGamutRGB, that breaks the "rule" and is LUT-based.
> I'd like to gain enough information that I could direct a file pointer
> to locations in the file, read a defined number of bits/bytes and
> interpret what's there.
I have no idea. :o)
> And...
>
> How complex is the math used in moving one color to another color space
> in one of the popular rendering intents? Are there examples of these
> equations available outside of proprietary technology?
With matrices, once the numbers in the matrix are defined, it's just
straight linear algebra, I believe.
With LUTs, I'm not sure exactly how the math works, but there are 16- or
8-bit tables depending on the direction of the transform, and a variable
number of grid points (say, 9 in A2B and 33 in B2A).
> I'd love to make a profile reader just for personal entertainment.
Sounds like a challenge!
Regards.
--------------
Marco Ugolini
Mill Valley, CA
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